This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous ink jet printers wherein a liquid ink stream breaks into droplets, some of which are selectively deflected.
Traditionally, digitally controlled color ink jet printing capability is accomplished by one of two technologies. The first technology, commonly referred to as xe2x80x9cdrop-on-demandxe2x80x9d ink jet printing, typically provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the print head and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
With thermal actuators, a heater, located at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble. This increases the internal ink pressure sufficiently for an ink droplet to be expelled. The bubble then collapses as the heating element cools, and the resulting vacuum draws fluid from a reservoir to replace ink that was ejected from the nozzle.
Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to vanLintel on Jul. 6, 1993, have a piezoelectric crystal in an ink fluid channel that flexes when an electric current flows through it forcing an ink droplet out of a nozzle. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
In U.S. Pat. No. 4,914,522, which issued to Duffield et al. on Apr. 3, 1990, a drop-on-demand ink jet printer utilizes air pressure to produce a desired color density in a printed image. Ink in a reservoir travels through a conduit and forms a meniscus at an end of an ink nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the nozzle, causes the ink to be extracted from the nozzle and atomized into a fine spray. The stream of air is applied for controllable time periods at a constant pressure through a conduit to a control valve. The ink dot size on the image remains constant while the desired color density of the ink dot is varied depending on the pulse width of the air stream.
The second technology, commonly referred to as xe2x80x9ccontinuous streamxe2x80x9d or xe2x80x9ccontinuousxe2x80x9d ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When print is desired, the ink droplets are directed to strike a print media.
Typically, continuous ink jet printing devices are faster than drop-on-demand devices and produce higher quality printed images and graphics. However, each color printed requires an individual droplet formation, deflection, and capturing system.
U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet.
U.S. Pat. No. 3,416,153, issued to Hertz et al. on Oct. 6, 1963, discloses a method of achieving variable optical density of printed spots in continuous ink jet printing using the electrostatic dispersion of a charged droplet stream to modulate the number of droplets which pass through a small aperture.
U.S. Pat. No. 3,878,519, issued to Eaton on Apr. 15, 1975, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, issued to Hertz on Aug. 24, 1982, discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a droplet formation point located within the electric field having an electric potential gradient. Droplet formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect droplets.
U.S. Pat. No. 4,638,382, issued to Drake et al. on Jan. 20, 1987, discloses a continuous ink jet print head that utilizes constant thermal pulses to agitate ink streams admitted through a plurality of nozzles in order to break up the ink streams into droplets at a fixed distance from the nozzles. At this point, the droplets are individually charged by a charging electrode and then deflected using deflection plates positioned the droplet path.
As conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes in which to operate. This results in continuous ink jet print heads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.
U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitude stimulations resulting in longer filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control of the break up points of the filaments and the placement of the air flow intermediate to these break up points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small, further adding to the difficulty of control and manufacture.
U.S. Pat. No. 4,190,844, issued to Taylor on Feb. 26, 1980, discloses a continuous ink jet printer having a first pneumatic deflector for deflecting non-printed ink droplets to a catcher and a second pneumatic deflector for oscillating printed ink droplets. A print head supplies a filament of working fluid that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an xe2x80x9cON/OFFxe2x80x9d type having a diaphragm that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is to be printed or non-printed. The second pneumatic deflector is a continuous type having a diaphragm that varies the amount that a nozzle is open, depending on a varying electrical signal received the central control unit. This oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time, being built up by repeated traverses of the print head.
While this method does not rely on electrostatic means to affect the trajectory of droplets, it does rely on the precise control and timing of the first (xe2x80x9cON/OFFxe2x80x9d) pneumatic deflector to create printed and non-printed ink droplets. Such a system is difficult to manufacture and accurately control, resulting in at least the ink droplet build up discussed above. Furthermore, the physical separation or amount of discrimination between the two droplet paths is erratic due to the precise timing requirements, increasing the difficulty of controlling printed and non-printed ink droplets and resulting in poor ink droplet trajectory control.
Additionally, using two pneumatic deflectors complicates construction of the print head and requires more components. The additional components and complicated structure require large spatial volumes between the print head and the media, increasing the ink droplet trajectory distance. Increasing the distance of the droplet trajectory decreases droplet placement accuracy and affects the print image quality. Again, there is a need to minimize the distance that the droplet must travel before striking the print media in order to insure high quality images.
U.S. Pat. No. 6,079,821, issued to Chwalek et al. on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and to deflect those ink droplets. A print head includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a receiving medium, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher. While the ink jet printer disclosed in Chwalek et al. works extremely well for its intended purpose, it is best adapted for use with inks that have a large viscosity change with temperature.
Each of the above-described ink jet printing systems has advantages and disadvantages. However, print heads which are low-power and low-voltage in operation will be advantaged in the marketplace, especially in page-width arrays. Commonly assigned, co-pending U.S. patent application Ser. No. 09/750,946 and Ser. No. 09/751,232, both filed in the names of David L. Jeanmaire and James M. Chwalek on Dec. 28, 2000, disclose continuous-jet printing wherein nozzle heaters are selectively actuated at a plurality of frequencies to create the stream of ink droplets having the plurality of volumes. A gas stream provides a force separating droplets into printing and non-printing paths according to drop volume. This process consumes little power, and is suitable for printing with a wide range of inks. However, the apparatus can have difficulty with registration of the ink droplets on the print media, due in part to slight deviations in the jet directions, and in part to slight variation in the gas flow velocity experienced by each droplet stream from jet to jet. Consequently, the droplets will not be registered to the same location on the receiver and a loss of image sharpness will occur, which is particularly evident in the printing of text. Therefore, it can be seen that there is an opportunity to provide an improvement to continuous ink jet printers. The features of low-power and low-voltage print head operation are desirable to retain, while providing high-speed printing, without a loss of image sharpness.
An object of the present invention is to provide for improved droplet placement in printers with print heads in which heat pulses are used to break up fluid into drops having a plurality of volumes, and which use a gas flow to separate the drops along printing and non-printing paths. This improved registration of printed droplets improves the quality of the image on the receiver media.
According to the present invention, an apparatus for printing an image comprises a print head having a group of nozzles from which streams of ink droplets are emitted. A mechanism is associated with each nozzle and is adapted to independently adjust the volume of the ink droplets emitted by the nozzle. Generally, two ranges of drop volumes are created at a given nozzle, with the first having a substantially smaller volume than the second. A droplet deflector is adapted to produce a force on the emitted droplets, said force being applied to the droplets at an angle with respect to the stream of ink droplets to cause ink droplets having the first volumes to move along a first set of paths, and ink droplets having the second volumes to move along second set of paths. An ink catcher is positioned to allow drops traveling along the first set of paths to move unobstructed past the catcher, while intercepting drops traveling along the second set of paths.
According to a feature of the present invention, an ink droplet forming mechanism is provided which is capable of slightly altering the size of the droplets having the first volumes, such that the droplet paths to the receiver are varied in a manner so that the printing droplets, corresponding to the printing of a line of image data, all strike the image receiver at the same point in the fast-scan printing direction.